The invention relates to anti-lockup protected vehicle brake systems with at least one wheel sensor for monitoring the rotational behavior of at least one vehicle wheel, with a circuit for obtaining the rotational speed of the wheel, with a reference velocity circuit for simulating the vehicle speed, with a circuit for providing a slippage control signal when the wheel velocity and vehicle reference velocity signals differ by a predetermined amount, and with a logic circuit for evaluating the slippage control signal and wheel acceleration and deceleration signals in order to control solenoid valves for the regulation of the brake pressure.
In anti-lockup protected vehicle brake systems, a disturbance may arise that causes the wheels of one axle to lock up while the wheels of another axle are controlled, thereby resulting in directional instability. For example, in so-called diagonal-individual regulation systems (DIR systems), which consist of only two control circuits that are arranged diagonally and in which case a sensor is located at the right wheel of the front axle and at the left wheel of the rear axle, and in which case the individual additional wheel(s) of the same axle are controlled by the regulated wheel, there exists the danger that in case of a malfunction leading, for example, to lockup of the rear axle, the vehicle will exhibit unstable motion behavior because the front axle in this case continues to be controlled.
A circuit for monitoring the duty cycle of the solenoid as a way of recognizing such troubles, is known, but has the disadvantage of a relatively large amount of time elapsing until recognition of the disturbance, so that the onset of unstable motion behavior cannot be prevented.
Further, the suggestion has already been made, to provide for a selective shutdown. In this way a minor reduction of the maximal engagement time of the solenoids can indeed be achieved, but the lower limit of the defect-recognition time is determined by the total wheel slippage period, which means that the time to shutdown in case of trouble is still so great that directional instability cannot be avoided.